mod_tls: Enable Prosody's certificate checking for incoming s2s connections (fixes #1916) (thanks Damian, Zash)

Various options in Prosody allow control over the behaviour of the certificate
verification process For example, some deployments choose to allow falling
back to traditional "dialback" authentication (XEP-0220), while others verify
via DANE, hard-coded fingerprints, or other custom plugins.

Implementing this flexibility requires us to override OpenSSL's default
certificate verification, to allow Prosody to verify the certificate itself,
apply custom policies and make decisions based on the outcome.

To enable our custom logic, we have to suppress OpenSSL's default behaviour of
aborting the connection with a TLS alert message. With LuaSec, this can be
achieved by using the verifyext "lsec_continue" flag.

We also need to use the lsec_ignore_purpose flag, because XMPP s2s uses server
certificates as "client" certificates (for mutual TLS verification in outgoing
s2s connections).

Commit 99d2100d2918 moved these settings out of the defaults and into mod_s2s,
because we only really need these changes for s2s, and they should be opt-in,
rather than automatically applied to all TLS services we offer.

That commit was incomplete, because it only added the flags for incoming
direct TLS connections. StartTLS connections are handled by mod_tls, which was
not applying the lsec_* flags. It previously worked because they were already
in the defaults.

This resulted in incoming s2s connections with "invalid" certificates being
aborted early by OpenSSL, even if settings such as `s2s_secure_auth = false`
or DANE were present in the config.

Outgoing s2s connections inherit verify "none" from the defaults, which means
OpenSSL will receive the cert but will not terminate the connection when it is
deemed invalid. This means we don't need lsec_continue there, and we also
don't need lsec_ignore_purpose (because the remote peer is a "server").

Wondering why we can't just use verify "none" for incoming s2s? It's because
in that mode, OpenSSL won't request a certificate from the peer for incoming
connections. Setting verify "peer" is how you ask OpenSSL to request a
certificate from the client, but also what triggers its built-in verification.

1 files changed, 4 insertions, 1 deletions

diff --git a/plugins/mod_tls.lua b/plugins/mod_tls.lua
index b240a64c..ac215b81 100644
--- a/plugins/mod_tls.lua
+++ b/plugins/mod_tls.lua
@@ -55,6 +55,7 @@ function module.load(reload)
 
 	module:log("debug", "Creating context for c2s");
 	local request_client_certs = { verify = { "peer", "client_once", }; };
+	local custom_cert_verification = { verifyext = { "lsec_continue", "lsec_ignore_purpose" }; };
 	local xmpp_alpn = { alpn = "xmpp-server" };
 
 	ssl_ctx_c2s, err_c2s, ssl_cfg_c2s = create_context(host.host, "server", host_c2s, host_ssl, global_c2s); -- for incoming client connections
@@ -67,7 +68,9 @@ function module.load(reload)
 
 	module:log("debug", "Creating context for s2sin");
 	-- for incoming server connections
-	ssl_ctx_s2sin, err_s2sin, ssl_cfg_s2sin = create_context(host.host, "server", host_s2s, host_ssl, global_s2s, request_client_certs);
+	ssl_ctx_s2sin, err_s2sin, ssl_cfg_s2sin = create_context(host.host, "server",
+		host_s2s, host_ssl, global_s2s, request_client_certs, custom_cert_verification
+	);
 	if not ssl_ctx_s2sin then module:log("error", "Error creating contexts for s2sin: %s", err_s2sin); end
 
 	if reload then